Dual-wavelength x-ray monochromator

ABSTRACT

A method for testing a surface includes finding respective first and second critical angles for total external reflection of radiation from an area of the surface at first and second wavelengths. The first and second critical angles are compared to determine an orientation of a tangent to the surface in the area.

FIELD OF THE INVENTION

[0001] The present invention relates generally to analyticalinstruments, and specifically to instruments and methods for thin filmanalysis using X-rays.

BACKGROUND OF THE INVENTION

[0002] X-ray reflectometry (XRR) is a well-known technique for measuringthe thickness, density and surface quality of thin film layers depositedon a substrate. Such measurements are particularly useful in evaluatinglayers deposited on semiconductor wafer substrates in the course ofintegrated circuit manufacture.

[0003] X-ray reflectometers are sold by a number of companies, amongthem Technos (Osaka, Japan), Siemens (Munich, Germany) and BedeScientific Instrument (Durham, UK). Such reflectometers typicallyoperate by irradiating a sample with a beam of X-rays at grazingincidence, i.e., at a small angle relative to the surface of the sample,near the total external reflection angle of the sample material.Measurement of X-ray intensity reflected from the sample as a functionof angle gives a pattern of interference fringes, which is analyzed todetermine the properties of the film layers responsible for creating thefringe pattern. The X-ray intensity measurements are commonly made usinga detector mounted on a goniometer. More recently, fast X-rayreflectometers have been developed using position-sensitive detectors,such as a proportional counter or an array detector, typically aphotodiode array or charge-coupled device (CCD).

[0004] For example, U.S. Pat. No. 5,619,548, to Koppel, whose disclosureis incorporated herein by reference, describes an X-ray thickness gaugebased on reflectometric measurement. A curved, reflective X-raymonochromator is used to focus X-rays onto the surface of a sample. Aposition- sensitive detector, such as a photodiode detector array,senses the X-rays reflected from the surface and produces an intensitysignal as a function of reflection angle. The angle-dependent signal isanalyzed to determine properties of the structure of a thin film layeron the sample, including thickness, density and surface roughness.

[0005] U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure isincorporated herein by reference, also describes an X-ray spectrometerbased on a curved crystal monochromator. The monochromator has the shapeof a tapered logarithmic spiral, which is described as achieving a finerfocal spot on a sample surface than prior art monochromators. X-raysreflected or diffracted from the sample surface are received by aposition-sensitive detector.

[0006] U.S. Pat. No. 5,740,226, to Komiya et al., describes a method foranalyzing X-ray reflectometric data to determine film thickness. Aftermeasuring X-ray reflectance as a function of angle, an averagereflectance curve is fitted to the fringe spectrum. The average curve isbased on a formula that expresses attenuation, background and surfaceroughness of the film. The fitted average reflectance curve is then usedin extracting the oscillatory component of the fringe spectrum. Thiscomponent is Fourier transformed to find the film thickness.

[0007] In order to obtain accurate measurements of film thickness, it isnecessary to precisely calibrate the angular scale of the reflection.Such a calibration requires, inter alia, exact control of the zero angleof reflection, so that the angle of the reflected beam relative to thesurface can be determined accurately. (In the context of the presentpatent application and in the claims, the term “zero angle” refers tothe orientation of a tangent to the reflecting surface at the point ofincidence of the radiation.) To make reflectometric measurements withoptimal accuracy, the zero angle at the measurement point should beknown to within 0.005°.

[0008] Although semiconductor wafers appear to be flat, in practicewafers typically deform slightly when held by a vacuum chuck duringproduction or inspection. The deformation is due both to the vacuumforce exerted by the chuck and to the weight of the wafer itself.Furthermore, the chuck may have imperfections, such as a slight bend inits axis, that cause deviations in the zero angle of the wafer as itrotates. As a result, inclination of the surface at different samplepoints on the surface of a wafer may vary by as much as 0.1-0.2°.Therefore, to perform accurate reflectometric measurements at awell-defined measurement point, it becomes necessary to recalibrate thezero angle at each point that is tested on the wafer surface.

SUMMARY OF THE INVENTION

[0009] It is an object of some aspects of the present invention toprovide improved methods and systems for reflectometry.

[0010] It is a further object of some aspects of the present inventionto provide methods and devices that enable rapid, accurate determinationof the zero angle of a surface under reflectometric inspection.

[0011] In preferred embodiments of the present invention, the zero angleof a surface under inspection is calibrated by measuring reflections ofX-ray beams from the surface at two different, known wavelengths, λ₁ andλ₂. The beams are aligned so as to impinge upon the surface at the samepoint and along substantially the same direction. Each of the beamsgenerates a reflectometric fringe pattern, which allows the criticalangle for total external reflection from the surface to be observed ateach of the two wavelengths. Even when the precise zero angle of thesurface is not known, the difference between the critical angles at thetwo different wavelengths can be measured with high precision.

[0012] In accordance with known physical principles, the critical angleat X-ray wavelengths is equal to a constant, k, which depends on thedensity of the reflecting surface layer, multiplied by the wavelengthitself. The precise measurement of the difference in the critical anglesat the two different measurement wavelengths can thus be used toaccurately calculate k with respect to the surface under inspection.Once k is known, the zero angle of the surface at the measurement pointis calibrated simply by subtracting kλ from the observed critical angleat either of the known wavelengths. The pattern of reflected fringes ateither or both of λ₁ and λ₂ can then be analyzed to accurately determinelocal surface properties including thickness, density and roughness ofthink film layers on the surface.

[0013] There is therefore provided, in accordance with a preferredembodiment of the present invention, a method for testing a surface,including:

[0014] finding respective first and second critical angles for totalexternal reflection of radiation from an area of the surface at firstand second wavelengths; and

[0015] comparing the first and second critical angles to determine anorientation of a tangent to the surface in the area.

[0016] Preferably, comparing the first and second critical anglesincludes taking an angular difference between the first and secondcritical angles, and calculating, based on the angular difference, aproperty of the surface for use in determining the orientation of thetangent. Most preferably, calculating the property includes finding aconstant k such that the angular difference between the first and secondcritical angles is substantially equal to |k(λ₂−λ₁)|, wherein λ₁ and λ₂are the first and second wavelengths, respectively, and setting kλ₁equal to the first critical angle so as to find the orientation of thetangent.

[0017] Preferably, finding the first and second critical angles includesirradiating the surface with first and second beams of the radiation atthe first and second wavelengths, respectively, wherein the first andsecond beams both impinge on the surface in the area along substantiallythe same direction. Further preferably, finding the first and secondcritical angles includes detecting the radiation reflected from thesurface using a common detector for the first and second beams. Mostpreferably, detecting the radiation includes detecting the radiation atthe second wavelength while preventing the first beam from impinging onthe surface. Alternatively, since the first and second beams haverespective first and second photon energies dependent on the first andsecond wavelengths, detecting the radiation includes discriminatingbetween the radiation detected at the first and second wavelengthsresponsive to the respective photon energies.

[0018] In a preferred embodiment, irradiating the surface includesgenerating the first and second beams using first and second radiationsources, respectively. Preferably, irradiating the surface includesfocusing the first and second beams onto the surface using first andsecond crystal monochromators, respectively, in mutually-adjacentpositions.

[0019] In another preferred embodiment, irradiating the surface includesgenerating the first and second beams using a single radiation sourcethat emits the radiation at the first and second wavelengths.Preferably, irradiating the surface includes focusing the first andsecond beams onto the surface using a single crystal monochromator forboth the first and second wavelengths. Most preferably, the secondwavelength is approximately equal to half the first wavelength, so thatthe crystal monochromator diffracts a first order of the first beam anda second order of the second beam toward the area of the surface.Alternatively, the crystal monochromator includes first and secondcrystal elements having respective first and second crystal spacings,selected so that the first crystal element diffracts the first beamtoward the area of the surface, while the second crystal elementdiffracts the second beam toward the area of the surface.

[0020] Preferably, finding the critical angles includes detecting anoscillatory pattern in the radiation reflected from the area as afunction of elevation angle relative to the surface, and the methodincludes analyzing the pattern, responsive to the orientation of thetangent, so as to determine a property of the surface. In a preferredembodiment, the surface has at least one thin film layer formed thereon,and finding the critical angles includes irradiating the surface withX-rays at the first and second wavelengths, and analyzing the patternincludes analyzing the X-rays reflected from the surface to determinethe property of the at least one thin film layer. In a further preferredembodiment, detecting the oscillatory pattern includes observing theoscillatory pattern at both of the first and second wavelengths.

[0021] There is also provided, in accordance with a preferred embodimentof the present invention, apparatus for testing a surface, including:

[0022] a radiation source, adapted to irradiate an area of the surfaceat first and second wavelengths;

[0023] a detector, adapted to receive radiation reflected from thesurface and to generate a signal responsive thereto; and

[0024] a signal processor, coupled to receive and analyze the signal soas to find respective first and second critical angles for totalexternal reflection of radiation from an area of the surface at thefirst and second wavelengths, and to compare the first and secondcritical angles to determine an orientation of a tangent to the surfacein the area.

[0025] Preferably, the radiation source is adapted to irradiate thesurface with first and second beams of the radiation at the first andsecond wavelengths, respectively, so that the first and second beamsboth impinge on the surface in the area along substantially the samedirection. Further preferably, the detector has a shape and size chosenso as to detect the radiation reflected from the surface in both thefirst and second beams, substantially without movement of the detector.Most preferably, the radiation source includes a filter, which isoperable to prevent the first beam from impinging on the surface whilethe detector detects the second beam.

[0026] There is additionally provided, in accordance with a preferredembodiment of the present invention, a crystal monochromator, includingfirst and second crystal elements, having respective first and secondcrystal spacings chosen so that the crystal elements diffract radiationincident thereon at respective first and second wavelengths at aselected Bragg angle, the crystal elements having a curvature chosen soas to focus the radiation at the first and second wavelengths to acommon focal area.

[0027] Preferably, the first and second crystal elements include firstand second crystals having respective front surfaces with the chosencurvature, positioned side by side so that the front surfaces define acommon curve. Alternatively, the first crystal element includes a bulkcrystal having a front surface with the chosen curvature, and the secondcrystal element includes a thin layer formed on the front surface of thefirst crystal element.

[0028] There is further provided, in accordance with a preferredembodiment of the present invention, a method for testing a surface,including:

[0029] finding respective first and second critical angles for totalexternal reflection of radiation from an area of the surface at firstand second wavelengths; and

[0030] comparing the first and second critical angles to determine aproperty of the surface.

[0031] There is moreover provided, in accordance with a preferredembodiment of the present invention, apparatus for testing a surface,including:

[0032] a radiation source, adapted to irradiate an area of the surfaceat first and second wavelengths;

[0033] a detector, adapted to receive radiation reflected from thesurface and to generate a signal responsive thereto; and

[0034] a signal processor, coupled to receive and analyze the signal soas to find respective first and second critical angles for totalexternal reflection of radiation from an area of the surface at thefirst and second wavelengths, and to compare the first and secondcritical angles to determine a property of the surface.

[0035] The present invention will be more fully understood from thefollowing detailed description of the preferred embodiments thereof,taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a schematic side view of a dual-wavelength system forreflectometry, in accordance with a preferred embodiment of the presentinvention;

[0037]FIG. 2 is a schematic top view of the system of FIG. 1;

[0038]FIG. 3 is a schematic top view of a dual-wavelength system forreflectometry, in accordance with a preferred embodiment of the presentinvention; and

[0039]FIGS. 4A and 4B are schematic, sectional views of curvedmonochromators used in dual-wavelength reflectometry, in accordance withpreferred embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0040] Reference is now made to FIGS. 1 and 2, which schematicallyillustrate a system 20 for X-ray reflectometry of a sample 22, inaccordance with a preferred embodiment of the present invention. FIG. 1shows a side view of the system, while FIG. 2 shows a top view. A firstX-ray source 24, typically an X-ray tube, emits a beam of X-rays at afirst wavelength λ₁, which is focused by a first crystal monochromator26 to irradiate a small area on sample 22. A second X-ray source 28, atanother wavelength λ₂, is focused by a second monochromator 30 toirradiate the same area. Any suitable X-ray tubes may be used for thispurpose, such as the XTF5011 tube, produced by Oxford Instruments ofScotts Valley, Calif. To generate the different wavelengths, the tubesused for sources 24 and 28 typically have different anode materials. Forexample, source 24 may have a copper anode and emit on the CuKa line(8.05 keV), while source 28 has a silver anode emitting on the AuLb line(11.44 keV). Alternative combinations of wavelengths will be apparent tothose skilled in the art.

[0041] Monochromators 26 and 30 preferably comprise curved crystalmonochromators, such as the Doubly-Bent Focusing Crystal Optic, producedby XOS Inc., of Albany, N.Y. Other suitable optics are described in theabove-mentioned U.S. Pat. Nos. 5,619,548 and 5,923,720, as well as inU.S. patent application Ser. No. 09/408,894, which is assigned to theassignee of the present patent application and whose disclosure isincorporated herein by reference. Although curved crystal monochromatorsare particularly convenient for implementing the present invention,other types and configurations of X-ray focusing and monochromatizingoptics may also be used, as will be apparent to those skilled in theart.

[0042] The X-rays reflected from sample 22 are received by a detector32. Preferably, detector 32 collects the reflected X-rays over a rangeof reflection angles between about 0° and 3°, both below and above thecritical angle of the sample for total external reflection at bothwavelengths λ₁ and λ₂. Detector 32 preferably comprises a detectorarray, such as a CCD array, as is known in the art. Details of the useof CCD arrays in X-ray reflectometry are described in U.S. patentapplications Ser. No. 09/409,046 and 09/833,902, which are assigned tothe assignee of the present patent application, and whose disclosuresare incorporated herein by reference.

[0043] A signal processor 34 analyzes the output of detector 32, so asto determine a distribution of the flux of X-ray photons reflected fromsample 22 as a function of elevation angle Φ at a given energy or over arange of energies. Processor 34 typically comprises a general-purposecomputer with suitable input circuits for receiving the detector output,and software for analyzing the reflected radiation intensity, asdescribed in the above-mentioned U.S. patent application Ser. No.09/833,902. Typically, sample 22 has one or more thin surface layers,such as thin films, so that the distribution of intensity as a functionof elevation angle exhibits an oscillatory structure, due tointerference effects among reflected X-ray waves from the interfacesbetween the layers.

[0044] Processor 34 analyzes the oscillatory structure of the reflectedintensity in order to determine the critical angle for total externalreflection from the surface of sample 22 at each of the wavelengths λ₁and λ₂. The oscillatory structure typically has a well-defined shoulder,corresponding to the critical angle, below which the reflectance of thesurface is nearly 100%. By finding the shoulder at both wavelengths,processor 34 identifies the critical angles, Φ_(crit)(λ₁) andΦ_(crit)(λ₂). These are relative values of the critical angles, sincethe zero angle at the measurement point on sample 22 is not yetprecisely known.

[0045] It is well known in the X-ray art that for any wavelength λ, thecritical angle is given by Φ_(crit)=kλ, wherein k is awavelength-independent constant (which depends on the square root of thedensity of the reflecting surface). Therefore, the difference betweenthe relative critical angles at the two measurement wavelengths is alsoproportional to k, i.e., ΔΦ=Φ_(crit)(λ₂)−Φ_(crit)(λ₁)=k(λ₂−λ₁).Processor 34 can thus compute k precisely based on the known differencebetween the irradiation wavelengths and the measured difference betweenthe relative critical angles at the two wavelengths. It then uses thisvalue of k to find the absolute value of Φ_(crit)(λ₁)−kλ₁. Bysubtracting the absolute value from the measured relative value of thecritical angle, processor 34 is able to reconstruct the zero angleposition exactly. Once the zero angle is known, the processor analyzesthe oscillatory structure of the reflections at λ₁ (and optionally atλ₂, as well) to determine properties of one or more of the surfacelayers of sample 22, preferably including thickness, density and surfacequality.

[0046] In order for the measurement of ΔΦ to yield an accurate value ofk, the X-ray beams at wavelengths λ₁ and λ₂ should impinge onsubstantially the same point on sample 22 along substantially the samedirection, without movement of the sample between the measurements atthe different wavelengths. For this reason, X-ray sources 24 and 28 andmonochromators 26 and 30 are preferably aligned, as shown in thefigures, so that the X-ray beams at wavelengths λ₁ and λ₂ are as nearlyas possible collinear. Assuming that source 24 is the primary source,which is used for subsequent reflectometric analysis of sample 22 (asdescribed above), monochromator 26 should have an effective aperturelarge enough to give a substantial signal at detector 32 over the fullrange of elevation angles of interest. The inventors have found that amonochromator with an azimuthal spread θ₁ of 0.85° is typicallysufficient for this purpose, with a range of elevations Φ₁ from 0° toabout 4.5°. (The angles are enlarged in the figures for clarity ofillustration.) On the other hand, if source 28 is used only to find thecritical angle at wavelength λ₂, there is no need to collect weak,high-angle reflection signals at λ₂. Lower collection efficiency istherefore acceptable at this wavelength. It is therefore sufficient formonochromator 30 to have a smaller aperture, typically with θ₂=0.25°,with Φ₂ ranging from 0° to about 0.6°.

[0047] Alternatively, the short-wavelength beam from source 28 may beused, as well, for measurements over a larger range of elevations. Inthis case, an oscillatory structure will also be observed in thereflections measured at λ₂. The short-wavelength oscillations are usefulin analyzing the properties of very thin films on sample 22, which maybe too thin to be detected effectively at λ₁.

[0048] In the configuration shown in FIGS. 1 and 2, the beams at both λ₁and λ₂ reflect from sample 22 and strike detector 32 side by side.Preferably, assuming detector 32 to comprise a linear array of detectorelements, with the array axis running vertically (in the view of FIG.1), the array elements are wide enough horizontally to capture bothbeams. In this case, sources 24 and 28 are preferably operated in closealternation, and the critical angle is measured at each of the twowavelengths in succession. Typically, if the signal at λ₂ is used onlyto determine the critical angle Φ_(crit)(λ₂), source 28 can operate foronly a short time, relative to source 24.

[0049] Alternatively, if the X-ray photon flux at detector 32 is low,the sources 24 and 28 may be operated simultaneously. In this case, thedetector signals at the two wavelengths are preferably distinguishedusing methods of energy discrimination known in the art. Because theX-ray photons at wavelength λ₂ are, in the present embodiment,substantially more energetic than the photons at λ₁, each photonincident on detector 32 at λ₂ will generate many more secondaryelectrons in the detector, resulting in a larger output pulse toprocessor 34. By distinguishing between the pulse heights, the processorcan separate the simultaneous signals at the two wavelengths.

[0050] Further alternatively, detector 32 may comprise a two-dimensionalmatrix array of detector elements. In this case, the column or columnsof detector elements at the left side of detector 32 (in the view ofFIG. 2) will detect the reflected X-rays at λ₂, while those at the rightside will detect the reflected X-rays at λ₁. In this case, too, sources24 and 28 can operate simultaneously.

[0051]FIG. 3 is a schematic top view of a system 40 for X-rayreflectometry, in accordance with another preferred embodiment of thepresent invention. This embodiment uses a single X-ray source 42 withtwo different emission wavelengths. Preferably, the emission wavelengthsλ₁ and λ₂ are chosen so that λ₁≅2λ₂. Diffraction by crystalmonochromator 26 is governed by the Bragg formula, i.e., 2dsinθ=nλ,wherein d is the crystal period, and n is the order of diffraction. Whenλ₁=2λ₂, monochromator 26 reflects and focuses λ₁ in its first order ofdiffraction at the same angle as it reflects and focuses λ₂ in itssecond order. This arrangement is advantageous in that the two X-raybeams that are used to irradiate sample 22 at λ₁ and λ₂ are inherentlyaligned, and the need for a second monochromator is eliminated.

[0052] To implement the embodiment of FIG. 3, for example, source 42 maycomprise an X-ray tube having an anode made of copper and strontium,preferably in proportions 80:20 Cu:Sr. The SrKb2 line, at 16.083 keV, isalmost exactly half the wavelength of the CuKa1 line, at 8.047 keV. (Inenergy terms, half the photon energy for SrKb2 is equal to only 5 eVless than the photon energy of CuKa1.) Given this small difference,monochromator 26 will focus both wavelengths efficiently onto thesample, while filtering out all other Cu and Sr wavelengths, with onlyan insignificant angular deviation between the beams in the azimuthal(θ) direction. The SrKb2 line, which is roughly 30 times weaker thanCuKa1, is preferably used only for finding the critical angleΦ_(crit)(λ₂). Alternatively, as noted above, the shorter-wavelengthradiation may also be used in observing an oscillatory pattern due to avery thin layer on the surface of sample 22.

[0053] As another example, the anode of the X-ray tube may comprisechromium and bismuth. The photon energy of the CrKa1 line, at 5.414 keV,is equal to only 4 eV less than half the photon energy of the BiLa1line, at 10.836 keV. Those skilled in the art will be able to find othersuitable line pairs, as well.

[0054] A filter 44, typically comprising a thick layer of aluminum, ispreferably used to block the CuKa1 radiation while making themeasurement at SrKb2. (Optionally, the SrKb2 radiation may similarly beblocked while the CuKa1 radiation is measured.) Alternatively, themeasurements at both wavelengths may be made simultaneously, usingenergy discrimination to separate the signals, as described above.

[0055] Alternatively, dual-wavelength source 42 may be configured toemit X-rays at two different wavelengths that are not multiples of oneanother, as long as the X-ray optics used to focus and monochromatizethe radiation incident on sample 22 are capable of handling bothwavelengths. Assuming a curved crystal monochromator is used, asdescribed above, this requirement can be met by assembling themonochromator from two different crystals, having respective spacings d₁and d₂, selected so that d₂/d₁=λ₂/λ₁.

[0056]FIG. 4A is a schematic, sectional view of a crystal monochromator50 designed in this manner for dual-wavelength operation, in accordancewith a preferred embodiment of the present invention. A first crystalelement 52, with spacing d₁ chosen for operation at wavelength λ₁, makesup approximately 90% of the area of monochromator 50. A second crystalelement 54, with spacing d₂ for operation at λ₂, makes up the remainderof the monochromator. As long as the proper ratio of the spacings d₁ andd₂ is maintained, and crystal element 54 has the same curvature ascrystal element 52, the two crystals will have the same focal point fortheir respective wavelengths.

[0057]FIG. 4B is a schematic, sectional view of a crystal monochromator60 designed for dual-wavelength operation, in accordance with anotherpreferred embodiment of the present invention. In this embodiment,crystal element 54 is formed as a thin layer over crystal element 52,which is a bulk crystal. This arrangement of the crystals is preferablycreated by growing the layer of crystal element 54 on a substrate ofcrystal element 52, using methods of thin- or thick-film depositionknown in the art.

[0058] Although the preferred embodiments described above make referencespecifically to X-ray reflectometry, the principles of the presentinvention may similarly be applied, mutatis mutandis, in other fields ofX-ray analysis. For example, the methods of the preferred embodiment maybe used to find the zero angle in X-ray diffractometry, as well as X-rayfluorescence (XRF) analysis, including particularly grazing emissionXRF. Grazing emission XRF is described, for example, in an article byWiener et al., entitled “Characterization of Titanium Nitride Layers byGrazing-Emission X-ray Fluorescence Spectrometry,” in Applied SurfaceScience 125 (1998), p. 129, which is incorporated herein by reference.The principles of the present invention may also be implemented inangle-sensitive detection systems for other energy ranges, such as fordetection of gamma rays and other nuclear radiation.

[0059] It will thus be appreciated that the preferred embodimentsdescribed above are cited by way of example, and that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and subcombinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofwhich would occur to persons skilled in the art upon reading theforegoing description and which are not disclosed in the prior art.

1. A crystal monochromator, comprising first and second crystalelements, having respective first and second crystal spacings chosen sothat the crystal elements diffract radiation incident thereon atrespective first and second wavelengths at a selected Bragg angle, thecrystal elements having a curvature chosen so as to focus the radiationat the first and second wavelengths to a common focal area.
 2. Amonochromator according to claim 1, wherein the first and second crystalelements comprise first-and second crystals having respective frontsurfaces with the chosen curvature, positioned side by side so that thefront surfaces define a common curve.
 3. A monochromator according toclaim 1, wherein the first crystal element comprises a bulk crystalhaving a front surface with the chosen curvature, and the second crystalelement comprises a thin layer formed on the front surface of the firstcrystal element.